1. Field of the Invention
The present invention relates to wireless communications. More particularly, the present invention relates to multi-input multi-output (MIMO) orthogonal frequency division multiplexing (OFDM) wireless communications.
2. Description of the Prior Art
Orthogonal frequency division multiplexing (OFDM) technique is known to have high spectrum efficiency and to be robust against inter-symbol interference (ISI) and fading caused by multi-path propagation. Another useful technique is bit-interleaved coded modulation (BICM), which has been widely used in OFDM systems. BICM is used between an encoder and a modulator for eliminating burst errors. Due to fast growth, existing wireless communication systems are not able to meet the demands for transmission bandwidth. Multi-input multi-output (MIMO) technique, by employing multiple transmit and receive antennas, is introduced to provide higher channel capacity which increases approximately linearly with the number of antennas used. Combining with MIMO structure, OFDM systems can further enhance the spectrum efficiency.
FIG. 1 is a block diagram illustrating a conventional MIMO OFDM system for BICM. Referring to FIG. 1, the conventional MIMO OFDM system 1 includes a transmitter 10 and a receiver 20. The transmitter 10 includes NT transmit antennas 18.1-18.NT, and the receiver 20 includes NR receive antennas 28.1-28.NR. An MIMO channel is formed among the transmit antennas 18.1-18.NT and the receive antennas 28.1-28.NR.
At the transmitter 10, information bits are encoded by a convolutional code (CC) encoder 11. The coded bits outputted from the CC encoder 11 may be punctured for variable code rates by a puncturer 12. The coded and punctured bits are parsed to multiple antenna streams by a parser 13 in a round-robin fashion. Each antenna stream is coped with a bit-level interleaver 14.p, a QAM mapper 15.p, a IFFT/GI modulator 16.p, an analog/RF circuit 17.p and a transmit antenna 18.p, where p represents the index of the transmit antenna and pε{1, 2, 3, . . . , NT}. It is noted that the QAM mapper 15.p and the IFFT/GI modulator 16.p form an OFDM structure.
The bit-level interleaver 14.p formats its input bits in a rectangular array of m rows and n columns as shown in FIG. 2. Referring to FIG. 2, the input bits {b1,b2, b3, L, bmn} are read in row-wise and read out column-wise by the bit-level interleaver 14.p. Then, the bit-level interleaver 14.p outputs the interleaved bits {b1, bm+1, b2m+1, L, bmn−m+1, b2bm+2, b2m+2, L , bmn−m+2, L, bmn}. Referring again to FIG. 1, the interleaved bits are converted into QAM symbols by the QAM mapper 15.p. The QAM symbols are then fed to the IFFT/GI modulator 16.p, arranged into OFDM symbols, and finally transmitted by the analog/RF circuit 17.p. 
At the receiver 20, an electromagnetic signal outputted from the transmitter 10 is passed through the MIMO channel and received by the receiver 20. Simply speaking, the receiver 20 is the reverse process of the transmitter 10. For example, the IFFT/GI modulator 16.p inserts guard interval (GI) and then implements inverse FFT (IFFT). However, the FFT/GI demodulator 26.q implements FFT and then removes GI, where q represents the index of the receive antenna and q ε{1, 2, 3, . . . , NR}. The bit-level deinterleaver 24.q formats its input bits in the same rectangular array as shown in FIG. 2, but its input bits are read in column-wise and read out row-wise.
For the equalizer 29, the commonly used equalizers include the zero-forcing (ZF) and minimum mean-square error (MMSE) equalizers. After equalization, the MIMO OFDM system 1 in each tone becomes multiple single-input single-output (SISO) systems. For the Viterbi decoder 21, a one-dimensional soft-bit demapper 25.q for each SISO system is also required. It is conceptually simple and easy to be implemented, but not optimal. This is because after equalization, noise components at the receive antennas 28.1-28.NR become correlated. Accordingly, the performance of the MIMO OFDM system 1 for BICM can be greatly degraded in typical MIMO channel conditions. A solution to the problem is to apply an optimal multi-dimensional soft-bit demapper. However, its computational complexity is very high.
Recently, a soft detector using a list sphere decoding (LSD) algorithm, called the LSD detector, has been proposed to replace the equalizer 29. The LSD detector provides a list of candidates allowing the bit metrics to be computed with lower complexity. Although the LSD detector can reduce the complexity of the optimal multi-dimensional soft-bit demapper, the computational complexity is still high, and the size of the memory required to save the candidate list is large, particularly when the number of the transmit antennas is large and when the size of the QAM mapper is large.
It would, therefore, be desirable to provide a MIMO OFDM system having lower computational complexity, particularly when the number of the transmitter antennas is large and when the size of the constellation mapper (e.g. QAM mapper) is large.